The invention relates to a planetary gear having a sun gear connected with a driven central shaft, a geared wheel, a group of first stepped planetary gears, a group of second stepped planetary gears, wherein all stepped planetary gears are supported on a common planet carrier, the number of the second stepped planetary gears corresponds to half the number of the first stepped planetary gears, the large step wheels of the second planetary gears engage the sun gear, the small step wheels of the second stepped planetary gears simultaneously engage one pair of adjoining large step wheels of the first stepped planetary gear and all small step wheels of the first stepped planetary gear engage the ring gear.
A planetary gear of this type has been described in the applicant's not published international application PCT/EP97/00288. The gear is designed as a so-called Wolfrom gear and, in particular, is very adequate for the production of very high total speed ratios with a good utilization of installation space. It further stands out by a high transmission quality, high efficiency and relatively low inertia moment, and by easy assemblage and low production costs.
U.S. Pat. No. 1,499,762 has disclosed a Wolfrom gear with stepped planetary gears in which the geared wheels have conical design and the planetary axles extend radially inclined toward the gear main axle.
Wolfrom gears are distinguished above all by high speed ratios with a very compact type of construction, but Wolfrom gears also have a few disadvantage basically conditioned. On the teeth (ring gears and the planets engaging the geared wheels) at the output are under a high torque load, elevated rotation speeds and relative speeds. This leads to elevated idle power requirements, reduced efficiency and under certain operating conditions to problems with vibration and noise. The planet carrier moving at relatively high rotational speed contributes to a substantial inertia moment of the gear. A high inertia moment produces high reaction torques in the event of abrupt changes in rotational speed.
Although compared to the Wolfrom gears, spur gear drives have more efficiency; they do not attain the great space and moment compactness of planetary gears. Other disadvantages of spur gear drives, compared to planetary gears, are that they are not well-suited to coaxial arrangement of input and output, and that pitch errors in the teeth of each wheel of a gear chain fully affect the transmission quality between input and output.
With conventional one-step planetary gears only with limitations, is it possible to implement very high speed ratios, due to problems with the tooth geometry in the very small gears needed therefor.
Already known planetary gears with several planet steps, connected in series, have a great number of parts and a large axial space.
The problem to be solved by the invention is to obtain high speed ratios with great space and moment compactness, to clearly improve the transmission quality and efficiency and at the same time to reduce the inertia moment and the tendency to vibration. In addition, the gear must be favorably produced and require no great assemblage expenditure.